This invention relates to a device for converting the rotation of a shaft to an electrical signal, typically known as a shaft angle transducer, and in particular, to a new and improved shaft angle transducer especially suited for use in a compass for a marine automatic pilot. A compass is used to provide an electrical input signal to an automatic pilot for a boat, with the signal indicating the heading of the boat. Desirably, the compass signal should be linear with angle over a wide range and have no zero dead band and no discontinuities. A compass typically comprises a card mounted on a pivot for rotation about a vertical axis, with the pivot permitting pitching and rolling of the boat. One or more magnets are mounted on the card for aligning the card with the earth's magnetic field. In order to avoid distrubing the operation of the compass, compass card position is typically read by optical systems. Typically the rotating compass card has a varying optical density and a lamp and photocell are positioned on opposite sides of the card, with the card varying the light transmission to the photocell as the card rotates on the pivot. Problems with manufacturing the variable density card and with accurate operation during pitching and rolling make this system unsatisfactory. In one variation, an opaque section of the compass card is eccentric with respect to the mounting and two photocells are utilized. However this configuration has not been satisfactory, particularly with regard to linearity.
Accordingly it is an object of the present invention to provide a new and improved shaft angle transducer which can provide an output linear with angular motion over a wide range, such as plus and minus 40.degree., without having a dead band and without having discontinuities in the output. A further object is to provide such a transducer wherein the linear characteristic is not adversely affected by pitching and rolling.